This disclosure relates generally to temperature sensing, and more particularly, to the use of new methodologies for interpreting distributed temperature sensing information.
Fiber optic Distributed Temperature Sensing (DTS) systems were developed in the 1980s to replace thermocouple and thermistor based temperature measurement systems. DTS technology is often based on Optical Time-Domain Reflectometry (OTDR) and utilizes techniques originally derived from telecommunications cable testing. Today DTS provides a cost-effective way of obtaining hundreds, or even thousands, of highly accurate, high-resolution temperature measurements, DTS systems today find widespread acceptance in industries such as oil and gas, electrical power, and process control.
DTS technology has been applied in numerous applications in oil and gas exploration, for example hydraulic fracturing, production, and cementing among others. The collected data demonstrates the temperature profiles as a function of depth and of time during a downhole sequence. The quality of the data is critical for interpreting various fluid movements.
The underlying principle involved in DTS-based measurements is the detection of spontaneous Raman back-scattering. A DTS system launches a primary laser pulse that gives rise to two back-scattered spectral components. A Stokes component that has a lower frequency and higher wavelength content than the launched laser pulse, and an anti-Stokes component that has a higher frequency and lower wavelength than the launched laser pulse. The anti-Stokes signal is usually an order of magnitude weaker than the Stokes signal (at room temperature) and it is temperature sensitive, whereas the Stokes signal is almost entirely temperature independent. Thus, the ratio of these two signals can be used to determine the temperature of the optical fiber at a particular point. The time of flight between the launch of the primary laser pulse and the detection of the back-scattered signal may be used to calculate the spatial location of the scattering event within the fiber.
DTS technology has been applied to production monitoring for identifying gas/oil entry depths along the wellbore. It is especially meaningful for open hole completion. Oil and gas is usually at geothermal temperatures. But it changes temperature as it approaches the wellbore due to pressure drop. This effect is usually called the ‘Joule Thompson’ effect. At steady state, a wellbore stays near geothermal temperatures except those depths, where oil and gas enters. DTS is used to try to find those depths that have different temperature from others.
Two methods are widely applied in industry to investigate these phenomena. DTS single trace analysis and DTS time-depth 2D image analysis. The first one is usually operated by including a limited amount of DTS curves in Depth-Temperature plot to find those noticeable local minimum temperatures on each single trace. The second method is to the DTS data in Time-Depth 2D plot. There is a need for better tools to address these phenomena.